[meta] Getter interface for primitive opaque types

[jpeletier]

This PR adds support for reading opaque types that map to primitives without having to invoke a serializer that may output an unwanted type.

The opaque interface now has "get" callbacks that allow to define a function that will retrieve the value from the opaque.

Opaques are now transparent 😅 to cursors.

  struct MyOpaque {
    int value = 0;
  };

  auto my_component = ecs.component<MyOpaque>();
  my_component.opaque(flecs::I32)
      // will be called when setting an int:    
      .assign_int([](MyOpaque *data, int64_t value) {
             data->value = value;
      })
      // **new** will be called when trying to read as an int:
      .get_int([](const MyOpaque *data) -> int64_t {
            return data->value;
      });

  MyOpaque instance;

  flecs::cursor cur(ecs, my_component, &instance);

  cur.set_int(79);
  std::cout << "value = " 
      << cur.get_int()  // **new** this now will work.  
      << std::endl;  // value = 79